The present disclosure relates to a method for preparing a surface of a YAG crystal for thermal bonding; a micro-chip device having a YAG crystal whose surface is prepared from the disclosed method; and a device for forming a metallization pattern on the surface of the YAG crystal.
Yttrium Aluminium Garnet (YAG) is represented by Y3Al5O12. YAG is a single crystal or crystalline ceramic material of the garnet group. YAG is generally used as a host material in various solid-state lasers. Some rare earth elements such as neodymium may be doped into YAG as active laser ions, yielding Nd:YAG lasers.
The recent advances in diode-pumped solid state lasers have facilitated extensive developments in the architecture of thermally manageable laser assemblies, including the micro-chip lasers. The micro-chip lasers are used as the major component in various laser proximity sensors and fuses. The laser components generally use Yb:YAG and Nd:YAG single crystals, as well as doped and undoped Y3Al5O12 polycrystalline ceramics or similar lasing glasses. These materials are effective in both industrial and defense applications.
A proximity fuse generally includes a micro-chip laser cavity, a laser diode source, and a heat sink. The micro-chip includes a Nd3+:YAG gain media layer (plate) and a Cr4+ YAG passive Q-switch media layer (plate). Both these layers (i.e., the Nd3+:YAG gain media layer and the Cr4+ YAG passive Q-switch media layer) are precisely polished to obtain the high targeting planarity and minimum surface roughness and are then bonded.
U.S. Pat. No. 5,563,899, which is hereby incorporated by reference in its entirety, discloses a diffusion bonding technique for YAG. The diffusion bonding technique for YAG and/or chemically-assisted optical contacting processes (for example, disclosed in “Optical Contacting: Changing the Interface of Optics” by Ch. Myatt et al.; Document #20060101; Precision Photonics Corporation; 2006, which is hereby incorporated by reference its entirety) are often used for assembling the identical, similar and dissimilar crystal, ceramic, and glass lasing materials. The diffusion bonding technique for YAG and the chemically-assisted optical contacting processes are physically (“hydrolyzes of oxides”) similar to each other. The diffusion bonding technique for YAG and the chemically-assisted optical contacting processes are different from each other in integration techniques and temperature-assisted processing. Also, both the diffusion bonding technique for YAG and the chemically-assisted optical contacting processes are equally applicable to integration of YAG and glass plates.
The micro-chip laser cavity is further formed by two dielectric coatings that are deposited on outer surfaces of Nd3+:YAG and Cr4+ YAG bonded plates. In general, the pump laser diode source generates 808 nanometer (nm) light beam. The light is then collimated in a collimated fiber output (or GRIN, or molded lens collimator) and delivered to the micro-chip laser cavity. The collimated light fiber output (before being delivered to micro-chip laser cavity 204) is illustrated in FIG. 2 by an arrow C. The light output from micro-chip laser cavity 204 is illustrated in FIG. 2 by an arrow D. The Q-switched (e.g., nanosecond pulse width or similar) pulsed light output at 1.06 μm wavelength is used in the sensor to define the distance to a moving target, which passes through a given space quadrant or sphere with the origin located in the center of the above micro-chip laser. This system, including the pumping light laser diode and the micro-chip laser, generates heat fluxes in laser diode and in micro-chip cavity. These heat fluxes have to be properly transferred, redistributed, and dissipated by means of an effective heat management device. The micro-chip laser assembly and laser diode are equipped with, for example, a Peltier cooler. This cooler and the attached heat spreader plate provide cross-plane mode heat sinking in the laser diode subassembly. The side surface of the cooler is also attached to the micro-chip laser and is configured to provide heat sinking of thermal flux that is generated in the laser micro-chip. With this arrangement, the second mode of heat transfer is primarily longitudinal.
In spite of simplicity of the above described design, the thermally-independent and precise sensing capability of the proximity fuse depends substantially on the ability of the thermal network to manage heat transfer in the laser diode and micro-chip cavity. This ability depends primarily on interfacial thermal resistance associated with the metal-oxide interfaces between YAG crystals or YAG ceramic components and metal heat spreaders. Also, the conventional interfacial conductance of dry and tight metal-oxide (ceramic) interfaces varies from 1,500 to 8,500 W/m2K. In the case of single crystal YAG interfacing a metal thermal spreader, the interfacial conductance of dry and tight metal-YAG interface can exceed 10,000 W/m2K. The diffusion bonding or optical contacting of YAG layers further induces interfacial thermal resistance at the interface of the essentially insulating Nd3+:YAG and Cr4+ YAG plates. The interfacial thermal resistance in the lattice disordered interface of Nd3+:YAG and Cr4+ YAG is also approaching 5,000-10,000 W/m2 K.
The diffusion bonding process of single crystal Nd3+:YAG and Cr4+ YAG plates is schematically illustrated in FIGS. 3A-E which show a four-step thermo-mechanically assisted interaction between the crystals due to their diffusion bonding. FIG. 3A shows the two crystals before optical contacting each other, and FIG. 3B shows the two mating units (crystals) optically contacting each other. The hydrolyses of mating oxide surfaces generally governs optical contacting. The intermolecular interaction is controlled by Van der Waals attractive forces and is characterized by weak interfacial strength. FIG. 3C shows formation of an actual contact between the mating units (crystals) at high temperature, T applied after the above optical contacting. FIG. 3D shows further activation of the mating surfaces of the two mating units (crystals or ceramic plates). As shown in FIG. 3D, the formed active centers typically cover 0.1% of total interface area, therefore limiting mass transfer processes through the interface. This in turn limits the lattice integrity of the bonded single crystal plates. In the case of identical or similar crystal materials, the major mass transfer mechanism is associated with the re-crystallization and co-sintering processes of the polished and nearly amorphous surface formations developed on the limited area active centers. These interactions are also weak, therefore causing imperfect interfaces reducing interfacial thermal conductance. In the case of ceramic materials, the major mass transfer mechanism is associated with the much more active grain boundary type re-crystallization and co-sintering involving not only the nearly amorphous surface formations but also the subsurface formations. The interfacial thermal conductance remains still limited. FIG. 3E shows the diffusion of dislocations and vacancies with insignificant grain boundary contribution on active centers. These lattice defects also contribute to the reduction of interfacial thermal conductance and strength. In all the diffusion bonding processes for single crystal and ceramic YAG, the mass transfer is limited by co-sintering of mating grains. Thus, the diffusion bonding of crystal and ceramic YAG plates further aggravates the heat transfer and thermal management in composite laser media.
In the case shown in FIGS. 3A-E, the mass transfer of YAG atoms is nearly impossible, while the dislocations and vacancies originated by the abusive grinding and polishing are easily accumulated at the plate interface. The process shown in FIGS. 3A-E assumes three major versions of optical contacting (wet, dry, and chemically assisted). In the case of optical contacting, the integration of plates leads to localized hydroxyl and van der Waals force bonds and are stimulated by active pressurization and heating. The bonds formed are characterized by limited strength and fracture toughness of interfacial and stress concentration, by moderate-to-high interfacial thermal resistance, and by the localization of the electromagnetic field on the imperfect interfaces. The diffusion bonding of lattice incoherent units leads to further weakened interfacial formations. Thus, the plate-bonded micro-chip lasers are further characterized not only by interfacial imperfections of macro-type but also by lattice compromised interface (micro-type). The compromised lattice interface is conventionally characterized by significantly decreased thermal conductivity due to dominating phonon confinement and scattering mechanisms. The micro-air pockets, the compromised planarity of mating interfaces, and several other factors further decrease the interfacial conductance. This aggravates thermal management and therefore prevents thermally-independent and precise sensing.
Currently available joining and packaging techniques for oxides and metals are often associated with the adhesive bonding, soldering and brazing processes. When these processes are conventionally applied to dissimilar material assemblies, the global and local mismatch thermal stresses are induced in the adherent laser crystals, ceramics, glasses, and alloys used in passive thermal spreaders and active coolers. The thermal processing that is associated with adhesive curing or soldering and brazing induces substantial thermal excursions and residual thermal stresses, as well as excessive and temperature-dependent displacements of adherent components. These factors lead to the temperature-dependent operations, further reducing the accuracy of sensing and fusing. Thus, there is a need for the precise, thermally manageable structural integration of crystal (or ceramic, or glass) laser components with various thermally conductive metals. This structurally strong joint should be formed from thermally compatible materials, and also be able to transfer and spread the heat fluxes.
One conventional system details the integration of active and complex Peltier-type cooler having a plurality of Peltier metal units with the ceramic substrates that are disposed to hold the Peltier elements. In this system, the integration of metal (Peltier units) with ceramic is achieved by the coating deposition of ceramic layers. The brazing is also proposed for further integration of the package. This process deals with the deposited dielectric ceramic layer that cannot be used as the lasing media in the application requiring high purity bulk crystal or ceramic YAG to transmit and amplify operational signal. Another conventional system details an optical system that includes a diode pump source and a thin disk gain media. An optical coupler is positioned between the diode pump source and the thin disk gain media to direct an output from the diode pump source to the thin disk gain media. This disk gain media is characterized by an anisotropic thermal expansion. The thermal mismatch with the first and second surfaces is therefore directional. The thermal compensation is provided by a special and approximate directional dicing (or cut) of the thin disk gain media, so that the thermal expansion mismatch is partially compensated by proper directional cut and by closely matched orientation of cooling surface. Both these conventional technical solutions do not appear to resolve the problem of thermally independent sensing and precise targeting. In the above discussed conventional systems, the global and local mismatch stresses were not minimized in a best possible manner. The differential (temperature-dependent) mismatches were also not minimized. In general, the differential thermal expansion mismatch between the constituent materials introduces a potential for feasible bow and de-lamination failure in the integrated laser assembly.
For almost all metal and oxide materials, the metal-oxide interfacial energies are conventionally characterized by very weak Van der Waals and electronic interactions. The intrinsic contact angles in these molten metals/YAG or glass pairs are larger than 90° . The polishing processes, abusing the surface of crystals and ceramics by abrasive particulates, and the inevitable roughness of the polished interface further increases the apparent contact angle and may lead to the formation of composite surfaces that are not fully covered by the deposited molten metal. Although the metallization and soldering of semiconductor laser diodes (e.g., InAs, InGaAs, etc) with heat sinking devices is a known problem, there is a need for improved wetting between the oxides, such as YAG crystal (YAG ceramic or glass) with metals.
The present disclosure provides improvements over the prior art methods for preparing a surface of YAG crystal for thermal bonding.